A long-time science-fictional notion, a high-speed train inside
an
evacuated tunnel, is attracting support in the form of "we
intend
to build it" projects. It is being called a "hyperloop". See the
link
for more details.

The main detail is that the degree to which the tunnel has its
air
removed by normal vacuum pumps, 99.9%, is still associated
with
significant air resistance when the speed of the train
approaches
1000 km/hr. I've seen some different ways to deal with it, like
compressing it at the front of the train and letting it loose
behind the
train (see second link, hah!).

This Idea is about something else. If a small amount of air
pressure in the tunnel is a problem, and cannot be removed
easily
by normal vacuum pumps, then what about an abnormal vacuum
pump? Let the train be the pump!

Let us suppose that at 600 kph, the train stars encountering air
resistance such that you have to start worrying about
aerodynamics and stuff like that, to minimize the problem.
Suppose instead there was a normal vacuum pump at the front
of
the train? It could capture some of that air pressure building up
in
front of the train, and store it on-board.

When the train docks at a passenger terminal, there will be air-
lock connections for boarding and disembarking. The train
could
now extend an umbilical to the wall of the tunnel, and pump out
the stored air. Now the overall tunnel is slightly more
evacuated
than it was before.

As all the trains in the tunnel do that simple thing, over and
over
again, the maximum speed in the tunnel can go up and up and
up,
with air resistance no longer being a problem.

Rescue Partyhttps://en.wikipedi...g/wiki/Rescue_PartyThis short story was written about 1945, and includes a train in an evacuated tunnel travelling under the ocean between continents. The idea is even older, however. [Vernon, Jul 20 2015]

I should note there are different notions for supporting the
weight of the train inside the tunnel. My personal
preference is magnetic levitation, which means it works
best if the tunnel is fully evacuated. The linked article
talks about how the train could in-essence fly inside the
tunnel, using that small remaining amount of air in the
tunnel --you would not want to remove that air, even
though the max speed is limited.